Recently, in magnetic disk devices, considerable progress has been made with regard to increasing capacity, providing high-speed operation, and reducing size, and the importance of such magnetic disk devices has been increasing as economical memory devices for computers.
A magnetic disk drive used in such a magnetic disk device is fixed in a sealed structure with packing for a hard disk drive (hereinafter abbreviated as “HDD packing”). The sealed structure prevents dust and contamination from entering, prevents malfunction and failure due to vibration, realizes high-density recording, a compact size, and size reduction, and ensures high reliability, etc.
Furthermore, HDD packing requires, as a function of packing, excellent properties such as flexibility, durability (in particular, hydrolysis resistance), a property of adhering to a base (in particular, a property of adhering to a metal), a low outgas property, and a mechanical strength besides a shape-retaining property after being applied onto a base.
Hitherto, for the purpose of preventing dust and contamination from entering and preventing failure due to vibration, a sealing member (i.e., packing) has been provided between a housing of a magnetic disk drive and the housing. For example, a porous material having elasticity, such as sponge or cork having a strip shape has been used.
However, in this method, such a strip-shaped porous material is bonded to a peripheral portion that contacts a cover of the housing with an adhesive by hand. Thus, the method requires a skilled operator and a large amount of labor, and therefore, is not suitable for mass production. In addition, this method essentially includes at least two steps of applying an adhesive and bonding a porous material such as sponge, and thus is complex and very inefficient. Furthermore, the housing is produced by molding a plastic. Therefore, when a thermosetting adhesive is used as the adhesive, the housing is deformed by heat, which is a serious problem in terms of production control.
To address this problem, recently, in the assembly and production of a magnetic disk drive and components thereof, an active-energy-ray-curable resin composition such as an ultraviolet-curable resin composition has been used for fixing and joining the components in order to meet requirements such as a reduction in work time and a reduction in cost which has been achieved by omitting a heating step.
For example, according to a known method for providing a sealing member on a housing for a magnetic disk drive, an ultraviolet-curable resin composition having elasticity is used as a sealing member provided between a housing for a magnetic disk drive and a cover of the housing, and the periphery of a surface that contacts the cover of the housing for the magnetic disk drive is coated with the ultraviolet-curable resin composition using a nozzle, and is then irradiated with ultraviolet rays to cure the coating (refer to, for example, PTL 1).
According to this method, an ultraviolet-curable resin composition is applied with a nozzle, and the resulting coating is then irradiated with ultraviolet rays to cure the coating, thus forming a sealing member having elasticity. Thus, dust and contamination can be prevented from entering and vibration can be prevented, and the sealing member can be easily and rapidly formed. Furthermore, since the ultraviolet-curable resin composition is applied with a nozzle, the thickness and the width of the sealing member can be adjusted to desired values.
However, the ultraviolet-curable resin composition used in PTL 1 generates a large amount of outgas due to a residual monomer after curing with ultraviolet rays. Accordingly, there may be a problem in that, for example, malfunction and failure tend to occur during recording and reading of a signal.
Therefore, in order to reduce a residual monomer, which is the main cause of the generation of outgas, heat treatment is usually conducted in advance under a high-temperature condition for a long time in preparation of a resin composition. However, this method has only a slight effect of reducing outgas and is not effective. This method has problems such as degradation and discoloration of the resin due to long-term heat treatment, and extremely poor productivity because this method requires labor and processing time.
A known method of producing a photo-curable sealing agent includes mixing, as essential components, at least one bifunctional (meth)acrylate represented by general formula (1) below:
(wherein R1 is a divalent aliphatic hydrocarbon group, whose main chain has 3 to 15 carbon atoms, and R2 and R3 each independently are a hydrogen atom or a methyl group),(B) at least one monofunctional (meth)acrylate represented by general formula (2):
(wherein R4 is an alkyl group having 6 to 20 carbon atoms and R5 is a hydrogen atom or a methyl group), (C) a styrene-isobutylene block copolymer, (D) a photoinitiator, and (E) a silica powder (refer to, for example, PTL 2).
It is described that the photo-curable sealing agent obtained by the above production method has good moisture permeation resistance and elasticity, and thus is useful as sealing of an electronic component casing that includes an electronic circuit element and an electronic component such as a hard disk therein, and a moisture-permeation-resistant sealing agent in other components that require moisture permeation resistance.
However, the photo-curable sealing agent described in PTL 2 also generates a large amount of outgas due to a residual monomer after curing by irradiation of ultraviolet rays as in the case of PTL 1. Accordingly, there may be a problem in that, for example, malfunction and failure tend to occur during recording and reading of a signal.
Furthermore, the following ultraviolet-curable composition for assembling a hard disk device is known: In the ultraviolet-curable composition used in the assembly of components of a hard disk device, a curable component of the ultraviolet-curable composition is a urethane (meth)acrylate obtained by an addition reaction between a hydroxyl group in a hydroxyalkyl (meth)acrylate and an isocyanate group in an isocyanate oligomer using either an organozinc compound or an amine compound as a catalyst, the isocyanate oligomer being prepared using an organozinc compound or an amine compound as a catalyst of an addition reaction between an isocyanate group and active hydrogen (refer to, for example, PTL 3).
It is described that this ultraviolet-curable composition does not contain a tin compound and does not contain an outgas component, and thus can be used for fixing or joining of hard disk device components etc. (e.g., packing of a housing case of a hard disk device, a cap seal of a spindle motor, fixing of a magnetic head, and fixing between a substrate and a connector).
However, the ultraviolet-curable composition obtained in PTL 3 has the following problems: (1) It is essential to conduct a curing process by irradiation of active energy rays for the purpose of ensuring a shape-retaining property. (2) Since the amount of a low-molecular weight prepolymer and the amount of residual monomer (e.g., diphenylmethane diisocyanate (MDI)) are large, the cross-linking density increases and the resulting cured product becomes excessively hard. (3) Since the amount of residual monomer after curing is large, a disk reading failure due to the generation of outgas tends to occur.
In addition, for example, acrylate compounds of oligomers have also been studied as resins used in active-energy-ray-curable resin compositions. However, regarding a reduction in the amount of outgas generated, satisfactory results have not been obtained.
As resins used as HDD packing, in addition to active-energy-ray-curable resins, various materials such as a rubber elastomer and polyurethane resin foam with an adhesive have also been studied. However, the amount of outgas generated from a formed product of all the materials is large. Accordingly, malfunction and failure frequently occur during recording and reading of a signal. Thus, the problem has not been solved. As a countermeasure for the problem, for example, degassing is performed by conducting heat treatment under a high-temperature condition in advance. However, this method has extremely low productivity, and a sufficient effect of reducing outgas cannot be achieved by this method.
Furthermore, recently, with the increase in the storage capacity of hard disks, the improvement in the accuracy of hard disks, and a marked increase in information processing speed, there has been a concern that a serious problem may occur in, for example, a hard disk drive having a large storage capacity even in the case where a very small amount of an outgas component adheres to a surface of a magnetic disk at a level that has not been hitherto considered as a cause of malfunction and failure.
As described above, when a very small amount of dust or contamination is mixed in a hard disk drive and adheres to a surface of a magnetic disk in the assembly and production, malfunction and failure may frequently occur during recording and reading of a signal. Therefore, regarding an active-energy-ray-curable resin composition such as an ultraviolet-curable resin composition used in the assembly of a hard disk drive, use of a resin composition in which the amount of outgas generated is small has also been strongly desired.
In a hard disk drive, a disk installed therein is rotated at a high speed, and thus components of the hard disk drive are joined and fixed to each other with a rubber-like elastic body capable of absorbing vibration that generates in the high-speed rotary movement. For this purpose, resins containing, as a main raw material, a urethane acrylate compound that forms a cured product having relatively high flexibility are generally used as active-energy-ray-curable resins used in joining and fixing such components.
However, urethane (meth)acrylate compounds that are generally used contain an organotin compound as a catalyst, and thus outgas components due to a residual monomer after curing is easily generated. The outgas components adhere to a disk surface, which tends to induce malfunction and failure during recording and reading of a signal. Hitherto, these malfunction and failure have often caused problems.
Adhesives that use an organic solvent, the adhesives having been hitherto used, have problems in terms of being hazardous to workers, risk of fire, environmental pollution, low drying rate, consumption of solvents, etc. Therefore, reactive hot-melt urethane adhesives have been actively studied as energy-saving environmentally friendly adhesives that are free of solvent and that do not require solvent recovery. Such reactive hot-melt urethane adhesives have been used as high-value-added products instead of existing solvent-based resins and water-based resins in wide applications, for example, adhesives for building materials and fibers, and coating agents.
The reactive hot-melt urethane adhesives have both a “hot-melt property” and a “moisture-curing property” and have been attracting as solvent-free adhesives in various fields. The “hot-melt property” is a property that the adhesive is solid at room temperature, the adhesive melts and becomes liquid or exhibits a viscous property when heat is applied thereto, and a cohesive force is again exhibited by cooling. The “moisture-curing property” is a property that adhesiveness is obtained by a cross-linking structure formed by a reaction between moisture (water) in air and an isocyanate group. Note that, in the present invention, moisture (water) and water vapor are treated as being the same.
A typical known example of such a reactive hot-melt urethane adhesive is an adhesive containing an isocyanate group-terminated urethane prepolymer.
However, in the case where such a reactive hot-melt urethane adhesive is used in the assembly and production of members for electronic devices, problems in terms of production control and quality control occur. Specifically, since the speed of a production line is high, a problem that the members are transferred to the next step while insufficiently joined and fixed by the reactive hot-melt urethane adhesive frequently occurs. In addition, there may also be a problem that, in general, a reactive hot-melt urethane adhesive is poor in terms of heat-resistant shape-retaining property because it is necessary to keep the adhesive in a molten state during application under heating.
As described above, there still remain problems in terms of properties such as a shape-retaining property after being applied onto a base, flexibility, durability (in particular, hydrolysis resistance), a property of adhering to a base (in particular, a property of adhering to a metal), a low outgas property, mechanical strength, and flame retardancy in the related art. Accordingly, an active-energy-ray-curable hot-melt urethane resin composition having balanced properties, a member for an electronic device and packing, the member and packing including the resin composition, have been strongly desired.